Scroll type fluid displacement apparatus are well known in the prior art. For example, U.S. Pat. No. 801,182 discloses a fluid displacement device which includes two scrolls each having a circular end plate and a spiroidal or involute spiral element. These scrolls are maintained angularly and radially offset so that both spiral elements interfit and make a plurality of line contacts between their spiral curved surfaces to thereby seal off and define at least one pair of fluid pockets. The relative orbital motion of the two scrolls shifts the line contacts along the spiral curved surfaces and, as a result, the volume of the fluid pockets changes. Since the volume of the fluid pockets increases or decreases dependent on the direction of the orbital motion, the scroll type fluid displacement apparatus is applicable to compress, expand or pump fluids.
In comparison with conventional compressors of the piston type, a scross type compressor has certain advantages such as fewer number of parts, and continuous compression of fluid. However, there have been several problems, primarily sealing of the fluid pockets and wearing of spiral elements; particularly since sealing of the fluid pockets must be balanced with wear of the contacting surfaces. It is desirable in a scroll type compressor to maintain sufficient sealing force at the line contacts because the fluid pockets are defined by the line contacts between the spiral elements, and the line contacts shift along the surface of spiral elements towards the center of spiral elements by the orbital motion of the scroll, to thereby move the fluid pockets to the center of the spiral elements with consequent reduction of volume and compression of the fluid in pockets. On the other hand, if the contact force between the spiral elements becomes too large in maintaining the sealing of the line contacts, wear of spiral elements' surfaces increases.
In particular, if the scroll type compressor is driven at high speed, the orbiting spiral element strongly pushes against the other spiral to thereby increase the driving force of the compressor and cause abrasion dust. This abrasion dust has a detrimental influence on the compressor or the refrigerating system used with the compressor. Mechanical efficiency of the compressor is also reduced by such abrasion dust.